Current foundry know-how does not enable a one-piece monocrystalline nozzle to be obtained. Thus, it is conventional to assemble two blades cast separately to form a nozzle. The assembly is generally carried out by a brazing/diffusion method, which is the most widely used method for the assembly of parts made of superalloy.
The brazing/diffusion technique consists in assembling parts, generally metal, using filler metal, in the form of powder or strip, placed between the parts to join together. The melting temperature of the filler metal is below that of the parts to join together. A first step, known as brazing, consists in subjecting the parts and the filler metal to a brazing temperature. The brazing temperature is determined such as to be below the melting temperature of the parts to join together. The increase in temperature to reach the brazing temperature is carried out in stages. The parts and the filler metal are then maintained at the brazing temperature for several tens of minutes. During brazing, an atomic diffusion takes place in a zone forming a brazed joint linking the two parts. A second step, called diffusion step, consists in carrying out a diffusion heat treatment in an oven, under controlled atmosphere for several hours, so as to assure sufficient homogenisation of the material situated at the level of the brazed joint.
In the light of the severe thermodynamic conditions to which the parts of a turbomachine are subjected in operation, the most suitable filler metal for assembling two blades is of composition NiCoSiB1060. The particularity of this filler metal is that it comprises silicon and boron which guarantee a good chemical bond between the blades to assemble.
However, a major defect of this component is the formation of zones of low mechanical and environmental resistances. Environmental resistance is taken to mean the capacity to withstand oxidation and corrosion. Yet, this type of both mechanical and environmental damage is very detrimental, leading to accelerated degradation of the brazed joint and leading to the disassembly of the two blades.
The rupture is generally located at less than one millimeter from the brazed joint, but rarely on the joint. This localisation is linked to diffusional phenomena on both side of the joint, which can exceed a hundred microns. The diffusion is notably exacerbated when the superalloy constituting the blades has grain joints, as is the case of polycrystalline superalloys.